Expansion is a known way to improve the filling power per unit weight of tobacco (usually measured in units of volume per gram of tobacco). One of the more practiced methods of expanding tobacco includes the steps of impregnating a charge of cut filler tobacco with an expansion agent (or "impregnant") and then rapidly heating the impregnated tobacco to volatilize the expansion agent, thereby causing an expansion of the tobacco tissue. The heating can be effected conveniently by entraining the tobacco in a stream of hot gas (or "tower gas") and directing the stream through a pneumatic conveying column ("tower"). In many expansion systems, a cyclonic separator located downstream of the tower separates the tobacco from the tower gas.
U.S. Pat. No. 3,771,533 discloses a process in which tobacco filler is impregnated with ammonia and carbon dioxide. The impregnated tobacco material is subjected to rapid heating, for example with a stream of hot air or air mixed with superheated steam, whereby the tobacco is puffed as the impregnant is converted to a gas.
U.S. Pat. No. 4,336,814 (PM 745) discloses methods for impregnating tobacco with liquid carbon dioxide, converting a portion of the impregnant to solid form and then rapidly heating the impregnated tobacco to volatilize the carbon dioxide and puff the tobacco.
U.S. Pat. Nos. 4,235,250 and 4,258,729 each disclose impregnation of tobacco with gaseous carbon dioxide under pressure and then subjecting the tobacco to rapid heating after a release of pressure.
U.S. Pat. No. 4,366,825 discloses a method of expanding tobacco in a flow of heated tower gas, with separation of the expanded tobacco from the gas stream being achieved in a tangential separator. The patent discloses a typical prior construction of a tower, wherein the pneumatic conveying column includes a vertically directed, cylindrical pipe.
U.S. Pat. No. 4,697,604 discloses another pneumatic conveying column comprising an upwardly inclined duct of rectangular cross-section. Inclined ducts of the type disclosed in this patent are generally disfavored, because their incline occupies extra floor space at manufacturing facilities, and because the inclined ducts allow gravity to urge tobacco particles toward the lowermost wall of the duct. The rectangular shape also presents corners, where localized eddies tend to entrap tobacco and toast (overheat) same. The corner regions exacerbate the risk of sparking (ignition) of the tobacco within the tower.
The more traditional, cylindrical, pneumatic columns are not without their own problems. Most troublesome has been the tendency of entrained tobacco to travel along one side of a conventional tower, instead of dispersing more uniformly amongst the tower gas. This flow phenomenon is inimical to achieving full and efficient expansion of the tobacco and is referred to in the art as "roping". The limited region along the tower where the tobacco is concentrated or roped is also referred to as a dense phase region. When roping occurs, a substantial portion of the pneumatic column remains as a gaseous region containing very little tobacco, and the concentrated tobacco directly interacts with only a limited portion of the gas stream passing through the tower, so that the heating of the bulk of the tobacco stream is not as rapid or effective as might be expected. A more complete expansion is achieved when tobacco is uniformly heated as rapidly as possible, beginning immediately at the lower portions of the column.
The problem of tobacco concentrating along the wall of a conventional tower seems to become more and more problematic as tower systems are made ever larger and/or as gas velocities in the conventional towers are reduced. A strong preference otherwise exists for the lower gas velocities, because they minimize pneumatic breakage of tobacco strands.
Production scale expansion towers can suffer a roping effect along their entire lengths, unless some corrective action is undertaken. We now believe that roping becomes especially problematic with the larger towers because of a perceived relationship between the diameter of a cylindrical tower and the endurance of a dense phase flow regime. The pipe diameter seems to be proportional with the length of pipe necessary for the dense phase flow to dissipate and for the mixing of tower gas and tobacco to reoccur. A cylindrical tower of a large diameter may therefore suffer roping along a greater portion of its length than a slimmer tower.
In the past, operators of large conventional expansion towers have attempted to limit roping by resorting to elevated gas velocities, which approach exacerbates breakage of tobacco and reduces dwell time of the tobacco within a given tower. The inclusion of baffling within expansion towers (known as "ski jumps") has also been attempted as a way to disrupt roping. However, such baffling also may exacerbate breakage and its effectiveness in disrupting roped flow has proven limited. A better solution has been sought and is herein disclosed, which does not exacerbate breakage and provides other advantages as will become apparent in the description which follows.